The present invention relates to a polishing pad used in a chemical mechanical polishing process, and more particularly to a method for forming micro-holes, perforations, or grooves on a polishing pad by a laser.
Generally, chemical mechanical polishing (CMP) is a high precision/mirrored surface polishing method used to obtain global planarization in a semiconductor device manufacturing process. In accordance with such CMP, a slurry is supplied between a polishing pad and a wafer to be polished, so as to chemically etch the surface of the wafer. Using the polishing pad, the etched surface of the wafer is mechanically polished.
Referring to FIG. 1, a typical CMP machine, which is denoted by the reference numeral 1, is schematically illustrated. Also, a CMP method using the CMP machine 1 is schematically illustrated in FIG. 2. The CMP method includes a chemical etching reaction process and a mechanical polishing process, which are conducted using a polishing pad 10 included in the CMP machine 1. The chemical etching reaction is carried out by a slurry 42. That is, the slurry 42 serves to chemically react with the surface of a wafer 30 to be polished, thereby making it possible for the mechanical polishing process, following the chemical etching reaction, to be easily carried out. In the mechanical polishing process, the polishing pad 10, which is fixedly mounted on a platen 20, rotates. The wafer 30, which is firmly held by a retainer ring 32, rotates while oscillating. A Slurry containing abrasive particles is supplied to the polishing pad 10 by a slurry supply means 40. The supplied slurry is introduced between the polishing pad 10 and the wafer 30. The introduced abrasive particles come into frictional contact with the wafer 30 by virtue of a relative rotating speed difference between the polishing pad 10 and the wafer 30, so that they conduct mechanical polishing. The slurry 42 is a colloidal liquid containing abrasive particles having a grain size of nanometers. This slurry 42 is spread on the polishing pad 10 during the polishing process. As the polishing pad 10 rotates during the polishing process, the slurry 42 supplied to the polishing pad 10 is outwardly discharged from the periphery of the polishing pad 10 due to a centrifugal force caused by the rotation of the polishing pad 10. In order to achieve an enhanced polishing efficiency, many abrasive particles should remain for a desirable lengthy period of time on the upper surface of the polishing pad 10 so that they participate in the polishing of the wafer. That is, the polishing pad 10 should make the slurry 42 be held on the surface thereof for as long a period of time as possible.
In order to make the slurry be held on the polishing pad for a long period of time, there may be used a method of forming spherical microcells having a size of micrometers (Em) or a method of forming perforations or grooves at the surface of the polishing pad. Such microcells, perforations and grooves act to control the flow and distribution of the slurry continuously supplied during the polishing process.
Conventionally, the formation of microcells at the polishing pad is achieved using a physical method or a chemical method. As the physical method, there is a method in which hollow microelements each having a cavity are incorporated in a polymeric matrix to form microcells. As the chemical method, there is a foaming method in which bubbles are chemically formed to form microcells.
The incorporation of microelements in a polymeric matrix is achieved by impregnating a large amount of microelements each having a cavity into a polymeric matrix in such a fashion that the microelements are uniformly distributed in the polymeric matrix, thereby forming microcells. The polymeric matrix is prepared by mixing a curing agent with a resin such as urethane, polyester, fluorinated hydrocarbon, or a mixture thereof. For the microelements, inorganic salt, sugar, water soluble gum, or resin is mainly used. Such microelements are made of polyvinylalcohol, pectin, polyvinyl pyrrolidone, polyethylene glycol, polyurethane or a combination thereof. Such microelements have an average diameter of about 150 Em. The microelements are uniformly distributed over the polymeric matrix in accordance with a high shear mixing process, so that they form uniform microcells. Referring to FIG. 3, microcells formed using the cavity bodies are illustrated. The pad formed with microcells in the above mentioned manner is subsequently cut into pieces each having a desired thickness to obtain a polishing pad. In each cut piece, microcells randomly distributed in the pad are opened at the cut surfaces of the cut piece, so that they are exposed in the form of a circular or oval cross section at the cut surfaces of the cut piece. The sizes and positions of the micro-cell cross sections exposed at the polishing surface of each polishing pad are random. Such random size and position of the exposed microcell cross sections serves to degrade a desired uniformity among polishing pads.
In accordance with the chemical method in which cells are formed using a foaming process, a polymeric matrix is formed by mixing a curing agent with a liquid-phase polyurethane forming substance having a low boiling point. Water or liquefied gas, which directly takes part in a chemical reaction to generate gas, is also used as a foaming agent, thereby producing bubbles to form cells in the polymeric matrix. The production of bubbles is achieved by way of a nucleation caused by a high shear mixing operation. A surfactant, which serves to achieve a reduction in surface tension, is also used to adjust the size of microcells, thereby achieving a desired uniformity of micro-cells. Microcells formed using the foaming process are shown in FIG. 4. Where cells are formed in accordance with the foaming process, however, there are problems in that the cells are too large to be applied to a CMP pad, that those cells have a non-uniform distribution, and that there is no method capable of adjusting the size and distribution of the cells.
The microcells formed using microelements each having a cavity or a foaming process have a spherical structure having a circular or oval cross-sectional shape. Due to such a shape, the microcells have a cross section varying in the thickness direction of the polishing pad. For this reason, the cross section of each microcell exposed at the polishing surface of the polishing pad is varied as the polishing pad is abraded during a polishing process. In other words, circular or oval microcells exposed at the polishing surface of the polishing pad are gradually reduced in diameter as the polishing process proceeds, and finally disappear. Eventually, microcells existing below the surface of the polishing pad without being exposed, are newly exposed at the polishing surface of the polishing pad.
Thus, the cross section of each microcell exposed at the polishing surface of the polishing pad is varied as the polishing pad varies in thickness during the polishing process. For this reason, there is a problem in that the polishing rate is non-uniform.
In order to form perforations or grooves at the polishing surface of the polishing pad, a mechanical machining method has been used which uses a cutting or milling process.
Referring to FIG. 5a, a cutter 70 for forming grooves is illustrated. When a polishing pad is machined using the cutter 70 mounted to a tool die on a lathe under the condition in which the polishing pad is rotated, grooves are formed on the upper surface of the polishing pad in the form of concentric circles, as shown in FIG. 5b. FIG. 5c is a cross-sectional view taken along the line Axe2x80x94A of FIG. 6b. Referring to FIG. 6b, an exemplary form of grooves formed using the cutter is illustrated. In FIG. 6b, the grooves are denoted by the reference numeral 75. An example of grooves having the form of concentric circles is disclosed in U.S. Pat. No. 5,984,769.
Referring to FIG. 6a, a horizontal milling machine 81 is shown, on which cutting saws 82 and spacers 83 are mounted. The cutting saws 82 are configured to move in an X-axis direction. A polishing pad 10 to be machined is moved in a Y-axis direction. In accordance with these movements, grooves 85 extending in a first direction are formed on the upper surface of the polishing pad 10. After the formation of the grooves 85, the polishing pad 10 is rotated 90xc2x0. In this state, grooves extending in a second direction orthogonal to the first direction are formed as the polishing pad 10 is moved in the Y-axis direction. Thus, grooves arranged in the form of a lattice are formed on the polishing surface, as shown in FIGS. 6b and 6c. 
Referring to FIG. 7a, perforating pins are illustrated which serve to form perforations at a polishing pad. When a perforating operation is carried out using the perforating pins under the condition in which the polishing pad is moved in a Y-axis direction, perforations are formed on the polishing surface of the polishing pad, as shown in FIG. 7b. An example of such perforations is disclosed in U.S. Pat. No. 5,853,317.
Since the conventional methods, which are used to form grooves or perforations at a polishing pad, utilize a cutting process conducted by a lathe or milling, the grooves have a fixed pattern such as concentric circles or a lattice. For this reason, it is difficult to form a groove pattern capable of effectively controlling the flow of a slurry. In accordance with the method for forming perforations using perforating pins, the perforations have a fixed shape. Also, the perforations are formed as the perforating pins are simply moved in an X or Y-axis direction. For this reason, the perforations have a simple and fixed pattern. Thus, it is difficult to obtain an effective hole pattern desired in a CMP process.
In a polishing pad machined to have grooves or perforations using mechanical means, debris formed during the machining process may be left in the grooves or perforations. Such debris may form scratches on a surface being polished during the CMP process.
Therefore, the present invention has been made in order to solve the problems involved with microcells formed using microelements each having a cavity or a foaming process and grooves or perforations formed using mechanical means. The present invention proposes a method capable of easily forming micro-holes, grooves or perforations having effective and diverse patterns on a polishing pad.
An object of the invention is to provide a method for forming micro-holes having the same function as microcells while having a controlled uniform distribution and a controlled uniform size, in order to eliminate the disadvantages involved with conventional methods using microelements each having a cavity or a foaming process, that is, a non-uniformity in the size and distribution of microcells resulting in a reduction or non-uniformity in the polishing rate of a polishing process.
Another object of the invention is to provide a method for forming grooves or perforations having diverse shapes, sizes, and patterns to effectively control the flow of a slurry during a polishing process, in order to eliminate the disadvantages involved with conventional mechanical methods, that is, insufficient control for the flow of the slurry due to a fixed shape or pattern of grooves or perforations.
In order to accomplish these objects, the present invention provides a method for forming, on a polishing pad, micro-holes, grooves or perforations having diverse patterns desired by the user, in accordance with a laser machining principle.
In accordance with an embodiment of the present invention, there is provided a method for fabricating a chemical mechanical polishing pad comprising the steps of: determining a pattern of micro-holes, grooves, or perforated holes to be formed on a polishing pad; inputting the determined pattern to a computer numerical control (CNC) controller; and driving a laser device adapted to irradiate a laser beam and a table adapted to conduct a three-dimensional movement and rotation while supporting the polishing pad, under a control of the CNC controller based on the inputted pattern, thereby irradiating the laser beam from the laser device onto the polishing pad supported by the table while moving the table in accordance with the inputted pattern, so that micro-holes, grooves, or perforated holes having a pattern corresponding to the determined pattern are formed on the polishing pad.